1. Field of the Invention
The present invention relates to a combustion-powered tool, and more particularly to a combustion-powered, fastener-driving tool for driving nails or other fasteners. In such a fastener-driving tool, liquefied gas contained in a gas tank is injected into a combustion chamber, where the liquefied gas is mixed with air and ignited. The power generated from this combustion drives a piston, which in turn drives the nail or other fastener into a workpiece.
2. Description of the Related Art
Combustion-powered tools of the type described above are disclosed in U.S. Pat. Nos. 4,483,474; 4,403,722; 4,522,162 and 5,592,580. A typical combustion-powered tool primarily includes a housing, handle, trigger switch, head cap, combustion chamber frame, push lever, cylinder, piston, driver blade, motor, fan, gas tank, spark plug, exhaust check valve, magazine, and tail cover. The head cap seals one end of the housing. The handle is fixed to the housing and includes a trigger switch, as well as a built-in battery. The combustion chamber frame is disposed inside the housing and is capable of moving in the lengthwise direction thereof. A spring urges the combustion chamber frame in a direction away from the head cap, but the frame is capable of opposing  the urging force of the spring to contact the head cap with an end nearest the same.
The push lever is movably disposed on the opposite end of the housing from the head cap and is coupled with the combustion chamber frame. The cylinder is fixed to the housing at a position enabling the cylinder to be in fluid communication with the combustion chamber frame for guiding the movement of the frame. Exhaust holes are formed in the cylinder. The piston is capable of sliding in a reciprocating motion in the cylinder. When the end of the combustion chamber frame contacts the head cap, a combustion chamber is formed by the head cap, the combustion chamber frame, the cylinder, and the end of the cylinder nearest the head cap. The driver blade extends from the side of the piston opposite the combustion chamber to the other end of the housing. The motor is supported on the head cap. The fan is positioned in the combustion chamber and fixed to the motor. When driven by the motor, the fan accelerates combustion by creating a turbulent flow with respect to combusted gas, non-combusted gas, and air in the combustion chamber. The fan also introduces outside air into the housing when the combustion chamber frame separates from the head cap to clear gas out of the combustion chamber frame and functions to cool the peripheral sides of the cylinder. The gas tank is accommodated in the housing and contains a  liquefied flammable gas that can be injected into the combustion chamber via a channel formed in the head cap. The spark plug is exposed in the combustion chamber for igniting the mixture of flammable gas and air. The exhaust check valve selectively covers the exhaust holes.
The magazine is disposed on the end of the housing opposite the head cap and accommodates nails or other fasteners. The tail cover is provided between the magazine and the push lever for supplying a fastener from the magazine to a position aligned with the driver blade.
In order to hermetically seal the combustion chamber when the combustion chamber frame contacts the head cap, a sealing member (seal ring) is provided one on a prescribed surface of the head cap that contacts the top part of the combustion chamber frame and one on the edge of the cylinder on the head cap side that contacts the bottom of the combustion chamber frame.
When the push lever is pressed against a workpiece, the combustion chamber is formed; liquefied gas from the gas tank mounted in the housing is injected into the combustion chamber; and the fan mixes air with the flammable gas. If the trigger switch is operated at this time, the spark plug ignites the gas-air mixture, causing explosive combustion. This combustion drives the piston and, consequently, the driver blade, to drive a nail into a workpiece, such as wood.  The combustion chamber frame is maintained in contact with the head cap for a prescribed time after the explosive combustion. After exhausting the gas, the exhaust check valve is closed to seal the combustion chamber, and a thermal vacuum is obtained on the combustion chamber side when a drop in temperature causes the pressure in the combustion chamber to drop. As a result, the piston rises due to the pressure differential above and below the piston.
The above-described conventional combustion-powered driving tool is involved with the following drawbacks.
(1) Pressing the push lever against the workpiece switches on a head switch (or push switch). The head switch (or push switch) actuates the motor, which drives the fan to rotate. When the user operates the trigger switch, the spark plug fires, igniting the gas-air mixture. However, if the trigger switch is operated in a relatively short time period after the motor and the fan begin to rotate, the motor and fan have not yet reached a rotational speed capable of producing a sufficient driving force. In such cases, a low driving force is produced.
There have been proposals for overcoming this problem that include use of an expensive low-inertia motor, or “coreless” motor, and methods for regulating the interval from the point that the head switch (or push switch) is turned on until the gas-air mixture is ignited. However,  these methods are expensive and greatly reduce work efficiency and user-friendliness.
(2) A secondary battery is used as a power source for driving the motor and igniting the sparkplug. An extra battery needs to be provided when the tool is used continuously or used frequently over a long period of time.
(3) When the tool is used under a low temperature circumstance, the liquefied gas injected into the combustion chamber is not sufficiently vaporized and thus cannot be mixed with air. In such a condition, explosive combustion does not occur even if the trigger switch is turned ON. Re-triggering the switch does not cause the explosive combustion to occur. The tool has to be separated from the workpiece and is again pressed against the workpiece to inject the liquefied gas into the combustion chamber. If this procedure is taken, explosive combustion may be taken place when the trigger switch is again turned ON. However, repeated injection of the liquefied gas consumes the gas in vain and the duration of time the tool is continuously usable with the loaded gas tank is shortened.
(4) Because a high voltage is applied to an electric circuit accommodated in the tool and a large current is flowing therein when the tool is operating, a voltage caused by noises is induced on the wiring of the tool, which prevents the tool from operating normally. 